Table 5. Chiral liquid crystal systems.
| Liquid crystal | Dopant | Sensing material | Detected analyte (gas or VOC) | Transduction method | Analyte interaction with sensing material | Refs. |
|---|---|---|---|---|---|---|
| CB CN COC |
– | CLCs polymera) cells and thin films | Alcohol and amine vapors | Optical (spectroscopy and naked eye) | Shortening of the helical pitch due to due to formation of H-bonds between the VOC and the matrix | [146] |
| Licritherm TM1013 | – | CLCs incorporated on a molecular imprinted polymer matrixb) | Methanol Ethanol Tetrahydrofuran Chloroform Tetrachloroethylene |
Optical (spectroscopy) and mass (quartz crystal microbalance) | Change in the helical pitch length due to VOC absorption in the system | [153] |
| CN CC |
– | CLC film on a glass disk with black reverse side | Acetone Benzene Hexane Pyridine |
Optical (spectroscopy) | Change in the helical pitch length due to VOC absorption in the system | [147] |
| CC COC CN |
– | CLC-coated styrene plate | Methanol Chloroform Tetrahydrofuran |
Optical (spectroscopy) | Distortion of the anisotropic phase resulting in a swelling of the helical pitch due to VOC interaction with the CLC | [148] |
| CC CCA CN |
– | CLC film coated side polished fiber | Tetrahydrofuran Acetone Methanol |
Optical (spectroscopy) | Change in the helical pitch length due to VOC interaction with the CLC | [151] |
| E7 | S1011 and DBD | CLC-coated PET films | Acetone Toluene |
Optical (POM & Spectroscopy) | Change in helical pitch length and a subsequent phase transition due to VOC interaction with the CLC | [154] |
| E7 | S1011, DBD, and CNT | CLC–CNT hybrid solution on rubbed PET films | Acetone | Optical (spectroscopy) and electrical | Change in helical pitch length and a subsequent phase transition due to VOC interaction with the system. Changes in the electrical resistance of the system | [159] |
| MBBA Cholesterol carbonate Cholesterol chloride Wacker oligomer |
Cholesterol carbonate Cholesterol chloride Wacker oligomer |
LCc) droplets on top of rubbed polyvinylalcohol-coated glass substrate | Cyclohexane Acetic acid | Optical (optical microscopy) | Phase transition of the CLCs in the presence of the VOC | [48] |
| Wacker oligomer | – | CLC dissolved in toluene and deposited on glass slide. Samples were heated up to 120 °C at the presence of air, pure water or solutions of polyacrylic acid | Water Ethanol Toluene |
AFM | Surface pattern reconstruction | [164] |
| E7 | Optically pure diamine-TADDOL derivative (R,R-1 and S,S-1) complex (1.) chiral binaphtyl dithiol derivative and a nonresponsive codopant – (R)-(+)-(1,1′-binaphthalene)-2,2′-dithiol (2.) | CLC thin film on planarly rubbed with a velvet cloth TAC foil or polyimide-coated glass slide | Carbon dioxide (1.) Oxygen (2.) | Optical (spectroscopy & naked eye) | Change in helical twist power of the chiral dopant due to reaction with the VOC | [162] |
| 1,4-Di(4-(6-acryloyloxypropyloxy) benzoyloxyl)-2-methylbenzene and 4-(4-(6-acryloyloxyhexyloxy) benzoyloxy) methoxybenzene | MAA | CLC polymericd) film printed on TAC foil | (1) Anhydrous TMA, (2) TMA in watersaturated nitrogen gas | Optical (spectroscopy and naked eye) | (1) Phase transition due to disruption of H-bonded polymer network upon exposure to the VOC, (2) Change in the helical pitch length due to adsorption of the VOC in the system | [149] |
| RM82 and RM105 | LC756 | Hydrogen-bridged CLC polymer networks with a porosity printed on TAC films | Alcohol vapors | Optical (spectroscopy) | Change in helical pitch length due to absorption of the VOC in the system | [155] |
| COC CN CB |
Dodecylamine | CLCs doped with dodecylamine pasted onto clean or DMOAP-coated glass slides (reverse side black). CLC doped films covered with PDMS followed by glass slides | Aldehyde vapors | Optical (spectroscopy & naked eye) | Color change due to a reaction between the dopant (dodecylamine) and the VOC | [160] |
| CN COC |
Oleic acid (1.) Oleyl amine (2.) Mono cholesteryl terephthaloyl chloride (3.) Cholesteryl phenyl hydrazide (4.) Cholesterol (5.) Cholesteryl chloroformate (5.) Methyl linolenate (6.) | CLC film onto thin Mylar film with black reverse side | Hydrochloric acid (1.) Hydrogen fluoride (2.) Hydrazine (3.) Unsymmetrical dimethyl hydrazine (4.) Nitrogen dioxide (5.) Nitric acid (6.) | Optical (naked eye) | Color change due to interactions with the VOC | [99] |
| E7 | R,R-TADDOLphenylhydrazine complex | LC-dopant mixture spin-coated onto polyimide-coated glass slides, previously rubbed with a velvet cloth | Acetone | Optical (spectroscopy) | Change in the helical twisting power due to a reaction between the dopant and the VOC | [161] |
| E E1e) | Magnetite NPs | LC doped with magnetite NPs absorbed into mesoporous alumina matrix | Carbon monoxide | Optical (spectroscopy) | Change in helical pitch length due to VOC interaction with dopant (magnetite NPs) and with the LC | [158] |
| E E1e) | Magnetite NPs | LC doped with magnetite NPs absorbed into silicon dioxide nanocomposite | Carbon monoxide | Optical (spectroscopy) | Change in helical pitch length due to VOC interaction with dopant (magnetite NPs) and with the LC | [157] |
| CLC-2103L | Magnetite NPs | LC doped with magnetite NPs into optically transparent porous material | Carbon monoxide | Optical (spectroscopy) | Change in helical pitch length due to VOC interaction with dopant (magnetite NPs) and with the LC | [156] |
| Schiff-bases (azomethine), Demus esters, tolans (diphenylacetylene), phenylcyclohecyls, and bicyclohexanes derived LC ZLI-1083 E7 5CB |
DDS-1015L & NYC-22133L | CLCs spin coated on a rubbed polyimidecoated glass substrate | Toluene Cyclohexane MEK Cyclohexane Acetone Ethanol Tert-butyl alcohol |
Optical (spectroscopy) | Change in helical pitch length upon interaction of the VOC with the CLC | [152] |
| CN COC CC |
LCR-262f) | LC films on structuresupporting polymersg)-coated glass substrate | Amine vapors | Optical (photometry and spectroscopy) | Change in helical pitch length upon interaction of the VOC with the CLC | [150] |
UV curable polymer Norland Optical Adhesive 61 (NOA61) and glass slide covered by another glass slide; separation distance between glasses kept by using two pieces of polyethylene spacer
Divinylbenzene (cross linker), styrene (monomers), AIBN (initiator) and tetrahydrofuran (solvent)
CLC mixtures composed of MBBA, derivatives of cholesterol and Wacker oligomer (glass-forming compound). CLC compositions → CLC-A: cholesterol carbonate (77%) and cholesterol chloride (23%); CLC-B: MBBA (59%) and CLC-C (41%); CLC-C: MBBA (80%) and Wacker oligomer (20%); CLC-D: CLC-A (90%) and Wacker oligomer (10%); CLC-E: CLC-A (80%) and Wacker oligomer (20%); CLC-F: CLC-A (65%) and Wacker oligomer (35%)
Other chemicals used for the CLC film: 1,4-Di(4-(6-acryloyloxypropyloxy)benzoyloxy)-2-methylbenzene (crosslinker), 4-(6-acyloyloxyhexyloxy)benzoic acid and 4-(6-acyloyloxyhexyloxy)-2-methylbenzoic acid (polymerizable benzoic acid derivative), Irgacure 369 (photoinitiator), hydroquinone monomethyl ether (thermal inhibitor) and tetrahydrofuran (solvent)
Chiral LC mixture containing multiple chiral cyano-biphenyl, cyano-terphenyl derivatives, with a chiral mesophase range of 282–318 K and optical anisotropy Δn 0.234 at 589 nm, 273 K
CB with a trifluoroacetyl receptor attached to a benzene moiety
PMMA and EG80A (polyurethane hydrogel) and hexamethyldisilazane. CB: Cholesteryl benzoate; CC: Cholesteryl chloride; CN: Cholesteryl nonanoate; CCA: Cholesteryl carbonate; COC: Cholesteryl olelyl carbonate; CLC: Chiral liquid crystal; DMOAP: (N.N-dimethyl-N-octadecyl-3-aminopropyl)trimethoxysilyl chloride; E7: Mixture of cyanobiphenyl and terphenyls; MAA: R(+)-3-methyladipic acid; NPs: Nanoparticles; PDMS: Poly(dimethylsiloxane); PET: Polyethylene terephthalate; TAC: Triacetyl cellulose; TMA: Trimethylamine.